FIG. 8 shows the structure of a known buried ridge-type self-aligned semiconductor laser. The semiconductor laser includes an n-type gallium arsenide (GaAs) semiconductor substrate 1 which has layers of aluminum gallium arsenide (AlGaAs) disposed on it to form the laser structure. An n-type Al.sub.0.5 Ga.sub.0.5 As first cladding layer 2, a p-type Al.sub.0.15 Ga.sub.0.85 As active layer 3, and a p-type Al.sub.0.5 Ga.sub.0.5 As second cladding layer 4 having a so-called forward mesa-shaped stripe ridge 4a are successively disposed on the substrate 1. The layers 2, 3, and 4 are grown, preferably by a metal organic chemical vapor deposition (MOCVD) process. An n-type GaAs current blocking layer 5 is disposed on the second cladding layer 4. A p-type GaAs contact layer 6 is disposed on the current blocking layer 5. Electrodes 7 and 8 are disposed on the substrate 1 and the contact layer 6, respectively. Laser oscillation takes place in the structure in a region 9 indicated in FIG. 8 by the broken line.
When a positive voltage is applied to electrode 8 relative to electrode 7 and a current is injected into the active layer 3 through the stripe ridge 4aof the second cladding layer 4, recombination of injected electrons and holes occurs in the active layer 3, generating light. When the bias voltage is raised to increase the injected current, the induced radiative component in the active region 3 is increased. When the light amplification gain due to the induced radiative component exceeds a threshold value determined by losses in the light waveguide of the laser structure, coherent light is radiated from the laser oscillation region 9.
FIG. 9(a) shows the relationship between the magnitude of the injected current and the magnitude of the light output of the laser of FIG. 8. At point A on the graph of FIG. 9(a), so-called catastrophic optical damage (COD) occurs. At that point, the facet of the laser through which light is radiated is damaged and may even be melted in localized areas. In order to increase the intensity of light at which COD occurs (the COD level), it is conventional to make active layer 3 relatively thin. Another known measure that increases the COD level is asymmetrical coating of the laser facets. The asymmetrical coating reduces the reflectance of the facet of the laser from which light is radiated and increases the reflectance of the opposite facet. The different reflectances are achieved by applying different coatings to the opposite laser facets.
These known steps for increasing the COD level both have disadvantages. In FIG. 10, the variation of threshold current with active layer thickness is plotted. When the active layer is made thinner, the threshold current for producing laser oscillation rapidly increases once a particular thickness, indicated as point B in FIG. 10, is reached. That thickness is about 0.05 micron, although some variation in the active layer thickness at which threshold current is mimimized is observed depending upon the laser structure. Because of the change in threshold current with the thickness of the active layer, the maximum improvement in COD level achieved by reducing the active layer thickness is limited in practice. The reduced reflectance of the front facet when asymmetrical reflectance coatings are applied results in undesirable optical feedback from external light sources and increases noise therefrom. As a result, the light output from the laser is excessively influenced by slight variations in the magnitude of the injection current.
FIG. 11 is a longitudinal cross-sectional view of another known semiconductor laser structure intended to increase the COD level. In that structure, light absorption in the neighborhood of the facet is reduced to raise the COD level. In FIG. 11, an n.sup.+ -type GaAs substrate 21 has disposed on it the layers forming the laser structure. An n-type Al.sub.0.35 Ga.sub.0.65 As first cladding layer 22, an Al.sub.0.05 Ga.sub.0.95 As active layer 23, and a p-type Al.sub.0.35 Ga.sub.0.65 As second cladding layer 24 are disposed on the substrate 21. Layers 22, 23, and 24 are successively grown by liquid phase epitaxy (LPE). A p-type GaAs current blocking layer 25 is disposed on a portion of the n.sup.+ -type GaAs substrate 21 at each of the facets of the laser. An n-type current blocking layer 26 is disposed on the p-type GaAs current blocking layer 25 adjacent layers 23, 24, and 25 at the facets of the laser. A p-type GaAs contact layer 27 formed by diffusing zinc into the GaAs is disposed on the p-type second cladding layer 24. A silicon dioxide (SiO.sub.2) insulating film 28 is disposed on n-type current blocking layer 26 opposite the substrate 21 and at the facets. Electrodes 29 and 30 are disposed on substrate 21 and on the contact layer 27 and the insulating film 28, respectively.
The laser of FIG. 11 is produced in two separate LPE growth steps. In the first step, the first cladding layer 22, the active layer 23, and the second cladding layer 24 are successively grown on the n.sup.+ -type GaAs substrate 21. Thereafter, a photoresist stripe is disposed on the second cladding layer 24, leaving the ends, i.e., the facets, exposed. The exposed parts of the structure are etched until the substrate 21 is exposed at the facets of the laser. The p-type current blocking layer 25 and the n-type current blocking layer 26 are grown on the partially completed device in the second LPE growth step. The SiO.sub.2 insulating film 28 is deposited on the n-type current blocking layer 26 and the contact region 27 is produced by diffusing zinc into the cladding layer 24 using the insulating film 28 as a mask. Finally, the electrodes 29 and 30 are deposited, thereby completing the device shown in FIG. 11.
In the laser device of FIG. 11, the current blocking layers 26 have a much larger energy band gap than that of the active layer 23. As a result, little coherent light generated at the active layer is absorbed in the current blocking layers at the facets, reducing the temperature rise for a particular current level, thereby increasing the COD level. This structure approaches a perfect, i.e., non-absorbing, mirror at the facets.
However, the laser of FIG. 11 has a number of disadvantages. The cost of producing the laser is too high to make it commercially practical. The active layer has, in the neighborhood of the facet, a regrowth interface that is exposed to air during the two step, i.e., two growth, production process. That exposure permits the formation of an oxide layer that results in rapid deterioration of the active layer at the facet and reduced laser lifetime. Furthermore, the surfaces of the active layer that are etched during the processing steps in manufacturing the laser device cause light refraction because of the creation of interfacial layers. As a result, the far-field pattern of the emitted light is greatly distorted.
Other theoretical phenomena that could increase the COD level are known but have not been successfully employed in practice. For example, current injection and surface recombination of charge carriers at the facet surfaces may be limited or eliminated. Current injection and surface recombination are accompanied by phonon emission, raising the temperature of the laser near the facets. By avoiding or reducing current injection and, thereby, phonon emission, the facet temperature for a particular current level is decreased, effectively increasing the COD level. However, like the structure of FIG. 11, structures exploiting these theoretical solutions have not previously been economically or practically successful.